Thermal insulation



United States Patent 3,338,686 THERMAL INSULATION Coultas D. Pears, Birmingham, Ala., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed May 11, 1965, Ser. No. 455,016 4 Claims. (Cl. 29-180) This invention relates generally to high temperature furnaces and more particularly to a novel thermal insulation which is adapted for use in such furnaces at temperatures up to 5000 F.

Such furnaces would find high utility in the evaluation of, refractory metals and other materials which would be contaminated in graphite type furnaces. As used hereinafter the term tungsten furnace is to be interpreted as a furnace employing tungsten metal heater elements.

An important design consideration in constructing high temperature furnaces is providing adequate thermal insulation therein to minimize heat losses. This is especially true for metallic, resistance-type furnaces which are adapted to operate over a wide temperature range and which, at thehigheroperating (temperatures 3500 F.), experience large heat losses. Heretofore, thermal insulation for such furnaces has generally consisted of a multi-wall construction with vacuum gaps between the respective walls designed to reflect the heat back into the center of the hot zone. Additionally, the use of such radiation shields requires a vacuum and such furnaces cannot be operated in an inert atmosphere without excessively large heat losses. While this design was not completely unsatisfactory, heat losses, through radiation transfer, remained high with a considerable amount of heat additionally being dissipated to the coolant.

Accordingly, it is a primary object of this invention to provide a novel thermal insulation comprising thin tungsten strips which are suitable for use in high temperature tungsten furnaces.

Another object is to provide a thermal insulation medium which may be employed in metallic, resistancetype, high temperature furnaces that are designed to reach operating temperatures to 5000 F. 1

Still another object is to provide a thermal insulation medium for high temperature metallic furnaces wherein heat losses are kept to a minimum, thereby permitting a more simple, compact and reliable design.

A still further object is to provide a thermal insulation medium which is suitable for high temperature use in vacuo and/or inert atmospheres.

In accordance with the present invention there is provided a novel thermal insulator for high temperature furnaces comprising thin tungsten strips. It is to be understood that the term strips as herein used is not limited to flat long pieces. Applicant has fund that tungsten metal, in the form of thin strips (one mil thick) is an excellent thermal insulator and, when incorporated in a high temperature furnace such as'a 5000 F. tungsten furnace would produce an overall temperature drop of about 2000 F. at 5000 F. across a oneinch thickness. The thermal effectiveness of these tungsten strips, which preferably consist of about a 4; diameter wound spirals, is achieved in a two-fold manner. First, the tungsten spirals provide a maximum number of shiny interfaces with a low emittance to minimize radiation losses and secondly, the tungsten spirals are disposed in a non-preferred orientation to minimize conduction losses.

The tungsten spirals may be utilized as a thermal insulation to contain the hot zone and reduce heat losses in any conventional high temperature furnace, such as a metallic and/or graphite furnace, providing the environment is compatible with tungsten. Where employed in graphite furnaces, the furnace would have to be restricted to temperatures below about 4970 F. which is the temperature at which tungsten carbide would be formed. It should be apparent that these tungsten strips would be highly suited in metallic furnaces which utilize a tungsten heating element such as a 5000 F. tungsten furnace. For a more complete description of such a 5000 F. tungsten furnace see TID21430. For this the spirals may conveniently be placed immediately around the hot zone in a suitable container of selected thickness. Where, for example, the hot zone is cylindrical, the container may comprise an outside wall formed from tantalum sheet and an inside wall formed from tungsten sheet with the tungsten spirals being disposed therein in the formed annulus. In such a high temperature furnace, which is designed to operate at temperatures to 5000 F. in an inert atmosphere or in vacuo the radial thickness required to reduce the heat losses below 25 kw. for a hot zone 2" in diameter by 6" long is about 1 /2 inches.

While the tungsten spirals may conveniently be 'employed in furnaces which utilize a vacuum and/or inert atmosphere interchangeably, certain procedures should be adhered to in order to preclude any deleterious attack on the spirals. In order to maintain the high degree of purity of the inert gas (when used), the furnace should be evacuated by a vacuum system during the heat-up period; this outgassing is designed primarily to remove essentially all contaminants, which cause embrittlement, deterioration and increase in emittance, from the furnace components. This preliminary step of hot outgassing is only mandatory Where the furnace components are thought to be contaminated with contaminants, such as lubricants, etc. On the other hand if the furnace has been constructed so as to be essentially free of such contaminants, then this step is not required. In any event where employed, this hot degassing need only be employed initially and/or after every major overhaul of the furnace.

In carrying out this outgassing step, it has been found that, although tungsten metal becomes contaminated at elevated temperatures above a vacuum of about 10* mm. of Hg, the tungsten spirals may be employed at elevated temperatures 3500" F.) without becoming contaminated, provided the system was initially evacuated below 10 mm. of Hg at or above the highest operating temperature contemplated.

The tungsten spirals may be formed, for example, from commercially. available tungsten metal strips (one mil thick). For this, long strips (0.001" x 0.5" x 12") are cut and rolled into wound spirals of a given diameter, such as /s" diameter. While it is preferred that the tungsten metal employed herein as thermal insulation be in the form of small diameter wound spirals, it is to be understood that other sizes and configurations are included within the scope of this invention, For example,

the thin tungsten strips may be stamped into a U 6011- figuration or bent into a S shape. While it has been found that there is no appreciable difference in thermal effectiveness between the different size tungsten spirals, it will be appreciated that, where the annulus in which the spirals are disposed is thin and/ or is not cylindrical, the smaller diameter spirals, i.e., the /3 diameter spirals, are capable of being more closely stacked and accordingly afford better results.

The temperature drop across a selected thickness of these tungsten spirals may be determined by any conventional temperature monitors. For example at temperatures below about 2400 F. the respective temperatures may be monitored by thermocouples; however at temperatures above 2400 F. the thermocouples were destroyed and the temperatures were monitored by an optical pyrometer. It should be noted that for these temperatures 2400 F.) the bottom plate temperature could not be obtained directly but was obtained from an extrapolation of bottom plate temperature versus heater wall temperature. This extrapolation was found to be essentially linear as shown in Table I below.

TABLE I Bottom plate Heater tube temperature, F. temperature, F. 500 100 1000 1600 1500 2100 2000 2600 2500 3120 3000 3650 3500 4175 4000 4700 Further illustration of the elfectiveness of this novel thermal insulation medium will be demonstrated in the following examples of which Example I demonstrates the eifectiveness of /s" diameter tungsten spirals and Example II the effectiveness A diameter tungsten spirals as thermal insulation.

Example I Apparatus was set up to determine the absolute effectiveness of /3" diameter tungsten spirals as a thermal insulation as follows: a graphite cup having approximately a one inch axial opening in its bottom was provided with a thin tungsten plate which covered the opening. One mil thick tungsten spirals (wound at about a .43" diameter) with thermatomic carbon were placed in the graphite cup in such a manner that the thermatomic carbon formed an outside annulus with the tungsten spirals in the center directly over the tungsten plate. In this manner the tungsten spirals filled a hole of approximately 1" in diameter and 1" long. A second tungsten plate was placed over the top of the tungsten spirals and Chromel-Alumel thermocouples were attached to both tungsten plates to measure the temperature drop across the tungsten spirals. The graphite cup was wrapped with graphite cloth and felt to assist the thermatomic carbon in minimizing the heat flow in the radial direction.

The assembled apparatus was lowered into a furnace and the bottom of the graphite cup positioned at the top of the hot zone. The temperature of the furnace was then stabilized at various temperatures and the temperatures of the heater tube and both tungsten plates recorded. At approximately 2400 F., the thermocouples were destroyed and the temperatures were monitored by an optical pyrometer. When the thermocouple of the bottom plate was destroyed, the bottom plate temperature was determined from the extrapolated plot of bottom plate temperaure versus heater tube temperature. The results are shown in Table 11 below.

TABLE II Temperature, F.

Heater Bottom Top Plate AT Across Plate Spirals 1 Temperature obtained from extrapolation of bottom plate temperature versus heater wall temperature.

2 Temperature monitored with optical pyrometer.

The extrapolation used in determining the bottom plate temperature was found to be not entirely accurate at the higher temperatures since the radiation transfer is much greater and the temperature of the bottom plate would approach the temperature of the heater tube, which would result in a larger temperature drop than recorded. Thus, the overall temperature drop produced by 1" thickness of diameter tungsten spirals would be over 2000" F. for a furnace operating at a temperature of 5000 F.

Example 11 The same apparatus and procedure employed in Example I was used to determine the absolute effectiveness of W diameter tungsten spirals. The results are shown in Table III below.

TABLE 111 T Temperature, F.

Heater Bottom Top Plate AT Across Plate Spirals 1 Temperature obtained from extrapolation of bottom plate temperature versus heater wall temperature.

2 A proper focus of the optical pyrometer was not obtained.

From the results shown in Table III, it may be seen that no appreciable difference in the temperature drop produced by the different size spirals was noted.

It is to 'be understood that the foregoing examples are merely illustrative and are not intended to limit the scope of this invention, but the invention should be limited only by the scope of the appended claims.

What I claim is:

1. An improved thermal insulation for high temperature furnaces comprising thin tungsten strips.

2. The improvement of claim 1 wherein said tungsten strips comprise short spirals wound at about a Ms diameter.

3. The improvement of claim 1 strips are about 0.001 inch thick.

4. In a high temperature, metallic, resistance type furnace adapted to be operated at temperatures to 5000 F. in vacuum or inert gas, an improved thermal insulation comprising one mil thick tungsten strips.

wherein said tungsten References Cited UNITED STATES PATENTS 1,157,288 10/1915 Blau 5 2 ,191,189 2/1940 Wade 3,212,864 10/1965 Rhudy 3,257,803 6/1966 Reid DAVID L. RECK, Primary Examiner.

RICHARD O. DEAN, Examiner. 

1. AN IMPROVED THERMAL INSULATION FOR HIGH TEMPERATURE FURNACES COMPRISING THIN TUNGSTEN STRIPS. 